A computational study of coherent mixing of electronic states using Ehrenfest-based non-adiabatic dynamics (with applications to hole dynamics and intersystem crossing)

Abstract

Most chemical processes can be adequately described by invoking the Born-Oppenheimer approximation wherein changes in electron distribution are assumed to be instantaneous relative to the motion of the nuclei. This thesis, and indeed much of photochemistry, is concerned with systems where this approximation breaks down - specifically near regions of strong electronic-nuclear coupling.

Since the electron distribution determines the chemistry of a system, if one could tweak the electronic state by populating a coherent superposition with a laser pulse, they could alter the chemical outcomes of reactions. Unfortunately these coherences are short-lived (the aforementioned electronic-nuclear coupling causes rapid decoherence within tens of femtoseconds) therefore exerting any electronic control would require even shorter pulses.

This thesis addresses two aspects of creating and understanding this coherence.

Firstly, we explore the dynamics of coherent superpositions of doublet cationic states populated by a (hypothetical) laser pulse corresponding to a hole in a bonding orbital of glycine. The resulting nuclear dynamics is controlled by a combination of the initial gradient and the instantaneous gradients arising from the oscillatory electron dynamics. To analyse the electron dynamics we use atom-projected spin densities, then using data analysis techniques, we can identify charge migration and charge transfer. Critically we can link the charge migratory modes and the nuclear dynamics, seen in the vibrational normal modes.

The second area we explore in this work is that of mixed singlet and triplet superpositions achieved by passing through regions of strong mixing of singlet and triplet surfaces in two small carbonyl-allylic systems. This effect is identifiable as intersystem crossing (ISC) - however, as the population transfers are partial, the ensuing dynamics is uniquely governed by the singlet/triplet admixture. This effect is particularly significant in ketene, where mixing of singlet and triplet states along the approach to a singlet/singlet conical intersection.